1. Field of the Invention
This invention relates to an arrangement of a peripheral circuit of a solid-state image device, and more particularly to a solid-state image device which includes a peripheral circuit having a non-volatile memory transistor, such as a transistor of the MNOS type, MONOS type or floating gate type, with a structure in which charge is trapped in insulation means below the gate electrode for varying the threshold voltage.
2. Description of the Related Art
An arrangement of a peripheral circuit in a conventional solid-state image device is described.
FIG. 1 shows a plan view of a conventional solid-state image device which includes a peripheral circuit having a non-volatile memory transistor such as a transistor of the MNOS type, MONOS type or floating gate type.
One of peripheral circuits of a solid-state image device having a transistor having such a structure as just described is a circuit which generates a voltage to be applied to a substrate. This circuit is called a substrate voltage generator. Normally, the substrate voltage is set to a voltage for controlling blooming of signal charge in a photo-electric transducer of a solid-state image element. The voltage value is different among different image devices, and it is required for the camera side to set the voltage for each image device. The substrate voltage generator converts a fixed voltage to be applied from the outside to a preset value for controlling blooming and stores charge in the non-volatile memory transistor arranged in a resistor divider to vary the threshold voltage in order to generate a desired voltage. An operation for trapping charge into such non-volatile memory transistors is performed in a testing step of the image device.
In the conventional solid-state image device described above, the non-volatile memory transistors of the peripheral circuits are arranged at suitable places inside an image circle to achieve overall compactness.
When a solid-state image device is applied to a camera, a circular image-formation plane (hereinafter referred to as image circle) 23, as shown in FIG. 1 is formed by an image lens. This image circle 23 is formed to cover the entire of image area (hereinafter referred to as image area) 22 of chip (solid-state image element) 21.
In the conventional example, although non-volatile memory transistor 24 of a peripheral circuit is not included in image area 22, it is included within image circle 23.
Therefore, if light strikes non-volatile memory transistor 24 in the circular image circle through the lens during use of the camera for a long period of time, then charge stored in insulation means below the gate electrode of non-volatile memory transistor 24 acquires energy. If this energy increases beyond the trapping energy level, then the charge is discharged from the trapping level. This gradually varies the threshold voltage of non-volatile memory transistor 24 and gives rise to a problem that a characteristic of the peripheral circuit is varied.